It is common practice to use a truck having a tiltable body portion and a tailgate to dispense or distribute granular material such as salt, sand, gravel or the like over a surface. One such use arises in road maintenance, especially in warm climates, where the "blacktop" surfaced roads become soft in the heat of the sun and must be periodically coated with a new layer of rather fine gravel, sometimes known as pea gravel. Other familiar uses are in dispensing salt or sand, or a mixture thereof, onto ice or snow coated road surfaces in colder climates; and in spreading sand or gravel in road construction or secondary road resurfacing wherein no blacktop or other "hard" surface is provided. Still other uses will be apparent.
When a common tiltable "dump truck" is used, it is possible to open the tailgate, which is normally pivotally mounted at the top rear portion of the load-carrying body, and to then tilt the body to cause the granular material to simply run out of the back of the truck as the truck is slowly driven forward. In order to control the flow of material, the chain (or two chains) being connected to the tailgage and hooked to one or more latch plates on the side walls of the body, thereby holding the tailgate open only a small amount and limiting the flow of material through the slot between the bottom of the tailgate and the rear edge of the body floor. In this context, the term "body" refers to the tiltable "box" part of the truck rather than the main frame of the truck.
As will be readily recognized, this manner of flow control is, at best, somewhat crude and ineffective. The slot between the tailgate and floor can be set at intervals which are determined by the size of the chain links, and there is no guarantee that it will remain the same, as the tailgate oscillates during truck motion. Thus, the layer of material deposited by the truck is usually not uniform and results in waste of material and in the use of additional manpower behind the truck to evenly spread the material. Also, when stopping, the truck body must be lowered and the tailgate closed to stop the flow of material usually leaving a pile of material at the stop point.
Further, because of the relative crudeness and inaccuracy of the slot width adjustment, granular materials of various average granule sizes cannot be handled with equal efficiency.
Because of these problems, various attempts have been made to develop devices, some of which are shown in the following U.S. Pat. Nos.: 1,966,122; Hunt, 1,981,095; Dickerson, 2,005,896; Hunt, 2,557,704; Stevens, 3,072,290; de Biasi.
However, these devices are relatively complex and difficult to construct and require special machinery for their fabrication.